The Amy3D and Amy3E genes are two sugar-regulated genes in the rice .alpha.-amylase family (Huang, N. et al., Nucleic Acids Res., 18(23):7077 (1990)). Amy3D is expressed in seedlings in response to sugar depletion and is not expressed when sugar, such as glucose, concentrations are elevated. Further, Amy3D is upregulated strongly in cell culture under conditions of sugar depletion or sugar deprivation. Unlike many other .alpha.-amylase genes, Amy3D is not induced in response to gibberellic acid.
Metabolic regulation of Amy3D expression provides a signal mechanism to help control sugar production in the source tissues of germinating cereal seedling. Thus, the rate of starch breakdown is modulated in response to the rate at which the embryo axis can utilize sugar for its growth.